The present invention is in the field of optical frequency conversion using birefringent nonlinear crystals, and in particular, relates to using an uncoated, specially cut lithium triborate (LBO) crystal.
Lasers operating in the infrared band are fairly common. Lasers in the visible light band are more difficult to produce and frequently involve the up conversion of lower frequency lasers by various nonlinear optical effects. Phase matching in birefringent nonlinear crystals used for optical frequency conversion generally requires a nonlinear interaction between three linearly polarized optical beams propagating in the same direction, but with one of the beams polarized orthogonally to the other two. Usually anti-reflection (AR) coatings are applied to the entrance and exit faces of the crystal, though it can be difficult or impossible to obtain ideal AR coatings for three wavelengths. These optical coatings on nonlinear crystals, however, can be an impediment to using them as frequency-conversion devices at high average power. For example, absorption by coatings on the biaxial crystal lithium triborate (LBO) causes local heating of both the coating and the crystal. Because LBO is extremely transparent at near-infrared and visible wavelengths, coating absorption can be larger than absorption in the bulk crystal. This heating, because of the large and anisotropic thermal expansion of LBO, may cause coatings to buckle or loosen.
From the foregoing it is apparent that there is a need for a frequency conversion means using birefringent crystals that does not involve optical coatings on the crystal faces that have the potential to overheat during high average power operation.
In a preferred embodiment, the present invention uses Brewster-cut uncoated planar surfaces on a specially cut birefringent crystal to obtain a third higher frequency by sum-frequency generation from a two-frequency input. The intersections of this specially cut nonlinear crystal are all convex, thereby greatly simplifying the required cutting and polishing.
The specially cut uncoated birefringent crystal has a primary optical axis along the X direction. Two different infrared wavelengths polarized along the Z-axis and traveling along the +X axis combine within the birefringent crystal by the sum-frequency process to generate a third, visible wavelength polarized along the Y-axis. The infrared input end has two surfaces: a lower triangular-shaped Brewster surface cut for Z-polarization transmission to pass the infrared input wavelengths and an upper inverted triangular-shaped Brewster surface cut for Y-polarization transmission. The infrared output end has a lower Brewster surface cut for Z-polarization transmission to pass the infrared wavelengths out of the crystal and an upper surface cut perpendicular to the lower surface. It is designed to reflect the Y-polarized visible light back along the xe2x88x92X direction in the upper portion of the crystal. An output coupler prism having an input Brewster surface cut for Z-polarization transmission is located adjacent to the Brewster cut surface at the infrared output end. This prism""s Brewster surface has anti-reflection coatings matched to the infrared input wavelengths and a high-reflection coating matched to the visible wavelength to pass the infrared and reflect the visible light. The visible wavelength traveling along the upper portion of the crystal in the xe2x88x92X direction passes out of the crystal at the upper Brewster surface cut for Y-polarization transmission.
Other aspects and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.